This invention relates generally to acoustic signal conditioning apparatus and more particularly to an inner decoupler for enhancing incoming signals incident on hull mounted sonar arrays while attenuating signals generated and emanating from the opposite side of the sonar array
Sonar comprises well known apparatus having both civilian and military applications. In order to provide signal enhancement of the incident signals, acoustic decouplers have been developed to improve the signal-to-noise ratio on hull mounted sonar arrays. In order to achieve this desired result, acoustic decouplers in the past have been designed to perform two functions, namely: (1) to provide, in conjunction with a signal conditioning plate, the proper impedance backing for one or more hydrophones included in the array; and (2) to isolate or decouple structure borne noise which emanates from a ship's hull and which tends to undesirably degrade the overall performance of the system.
With regard to the first function, an ideal signal conditioning device is one which when placed directly behind the hydrophones operates to enhance the signal response at all frequencies without introducing phase shifts. In known prior art apparatus, thick steel plates having pressure release, i.e. low impedance, backings have been used to approach this end. However, as the need for improved performance requires the use of lower and lower operating frequencies, the thicknesses and weight requirements for the steel plate structures become prohibitive from a practical standpoint and alternative means have been resorted to obtain signal conditioning, i.e. signal enhancement
A fluid layer has also been known to be used as a stand-off between a signal conditioning plate and the hydrophones. The mass of the fluid layer may be traded for the mass of the signal conditioning plate with some benefit in low frequency performance; however, the frequency bandwidth over which signal gain may be obtained will be reduced. In the extreme case where fluid mass totally replaces the mass exhibited by the signal conditioning plate, the low impedance backing is usually positioned at a one-quarter wavelength distance from the hydrophone to maximize signal gain. The main disadvantage of this approach is that the bandwidth is seriously curtailed compared to that obtained with other alternatives.
With respect to the second function, low impedance devices of diverse forms and types are known and have been used extensively to decouple structure borne noise from the sonar array. Nevertheless in applications where resistance to hydrostatic pressure is a factor, acoustic compliance should be maintained under hydrostatic pressure. However, this function becomes evermore difficult as one resorts to ever lower frequencies of operation.
Many different devices have been proposed and/or considered to provide signal conditioning for sonar arrays. The great majority require the use of a low impedance termination to provide a high transmission loss to the system. Basic types include absorptive baffles, compliant tube baffles and sound diffusers.
Absorptive type baffles remove unwanted energy from the system and, as is well known, come in many varieties. Conventional rubber/air absorbers are very effective in their design pressure/frequency/temperature range, but are usually pressure sensitive and are otherwise limited in their frequency response. Absorbers which provide a gradual impedance transition from front to back through geometric shapes such as wedges or cones, are also well known. For low frequency applications, however, the dimensions of the wedges which would be required for effective signal conditioning exceed practical limits. Several other versions of gradual impedance transition coatings have been widely proposed, but their limitation is one of size relative to an acoustic wavelength for the lowest frequency of interest.
With respect to compliant tube baffles, such elements utilize the mechanical resonances of squashed metallic tubes encased within viscous elastic matrices to attain dynamic compliance. Compliant tube baffles have the advantage that they are essentially pressure insensitive. Their major limitation is their excessive weight, and in some instances, fatigue cracking of the tubes result when subjected to pressure cycling or shock tests. Other drawbacks include the high manufacturing costs involved, difficulty in installation, and the narrow frequency coverage achieved thereby.
Other known approaches for achieving signal conditioning involve the use of sound diffusion through multiple scattering to make noise more random. Undesired sound noise can either add coherently or randomly to an acoustic signal. In particular, structure borne noise is detrimental if its discrete frequency components add coherently, i.e., in phase, to the acoustic signal being detected. This requires complex electronic detectors to discriminate against coherent noise interference. On the other hand, random noise can be dealt with through time averaging or other standard signal processing techniques. Signal processing is burdensome, however, and thus comprises an undesired approach.
Accordingly, it is an object of the present invention to provide an improvement in underwater communication and detection apparatus.
It is a further object of the invention to provide an improvement in sonar apparatus.
It is another object of the invention is to provide an acoustic decoupler for a sonar array which combines both a signal conditioning function as well as a structure borne noise decoupling function.
And it is still a further object of the invention to provide an improved sonar array of a simple and efficient design for improving the signal-to-noise ratio of hull mounted arrays.